Avian influenza has long been known to circulate in wild aquatic birds of the orders Anseriformes (ducks, geese, and swans, etc.) and Charadriiformes (shorebirds, gulls, and auks, etc.) - and importantly - these species can often carry these viruses will little or no ill effect.
All influenza A viruses - including human and swine strains – are believed to have originated from these natural avian hosts.
Less well adapted to the viruses, and therefore far more susceptible to illness, are the Galliformes; an order of heavy-bodied ground-feeding birds that includes turkey, chicken, quail and pheasant. Commercially raised gallinaceous birds are easily infected by both low and high path viruses, and can rapidly succumb to HPAI infection.
Although it is no secret that Passerine birds (aka `perching’ and `songbirds’) have been infected with HPAI H5N1 in the past (see EID journal Characterization of Avian Influenza Viruses A (H5N1) from Wild Birds, Hong Kong, 2004–2008), until recently little research has been conducted on how highly pathogenic avian influenza affects this largest of all classification of birds.
Because of the low expectations of finding HPAI H5 in non-aquatic birds, passerine birds are rarely routinely tested in North America (see Surveillance Plan for the Early Detection of H5N1 Highly Pathogenic Avian Influenza Virus in Migratory Birds: 2009).
Nevertheless, there has been a growing concern that passerine species could be a significant vector for HPAI viruses, and might contribute to local transmission of these viruses to poultry flocks.
We’ve seen the deaths of passerine birds like Oriental Magpie Robins in Hong Kong, and Crows in India, infected with HPAI H5 viruses, and a little more than a month ago, in Minnesota DNR: HPAI H5 Discovered In Second Wild Bird, we saw the first detection of North American HPAI H5 in a passerine species (Chickadee).
The Survey of H5N1 Flu Virus in Wild Birds in 14 Provinces of China from 2004 to 2007, published in 2009, found 26 positive samples distributed across 9 species from among 7320 passerine samples tested, providing a very low incidence of 0.36%. However, among tree sparrows tested, the prevalence was three times higher at 1.09%.
The 2012 PLoS One Study A Survey of Avian Influenza in Tree Sparrows in China in 2011, found serological evidence of prior H5 subtype HPAI infection in 94 of 800 (11.75%) of sparrows tested, showing that HPAI H5 infection need not always prove fatal in that species.
In 2013, in Pathogenesis in Eurasian tree sparrows inoculated with H5N1 highly pathogenic avian influenza virus and experimental virus transmission from tree sparrows to chickens by Yamamoto Y, Nakamura K, Yamada M, Mase M., the authors found that inoculated birds lived up to 11 days and shed copious amounts of the virus during that time, and suggested that Eurasian tree sparrows could be potential vectors to housed poultry.
All of which serves as prelude to a study, published on Friday, that looks at the susceptibility of three passerine bird species (reed buntings, brown-eared bulbuls and pale thrushes) to HPAI H5N1, and finds that 2 out of 3 (buntings & bulbuls) sickened and died, while pale thrushes seroconverted exhibited no clinical signs of infection.
Avian Pathol. 2015 Aug;44(4):243-247.
Fujimoto Y1, Usui T, Ito H, Ono E, Ito T.
Highly pathogenic avian influenza (HPAI) viruses of the H5N1 subtype have spread throughout many areas of Asia, Europe and Africa, and numerous cases of HPAI outbreaks in domestic and wild birds have been reported. Although recent studies suggest that the dissemination of H5N1 viruses is closely linked to the migration of wild birds, information on the potential for viral infection in species other than poultry and waterfowl is relatively limited.
To investigate the susceptibility of terrestrial wild birds to infection with H5N1 HPAI viruses, common reed buntings (Emberiza schoeniclus), pale thrushes (Turdus pallidus) and brown-eared bulbuls (Hypsipetes amaurotis) were infected with A/mountain hawk-eagle/Kumamoto/1/07(H5N1) and A/whooper swan/Aomori/1/08(H5N1). The results showed that common reed buntings and brown-eared bulbuls were severely affected by both virus strains (100% mortality).
While pale thrushes did not exhibit any clinical signs, seroconversion was confirmed. In common reed buntings, intraspecies-transmission of A/whooper swan/Aomori/1/08 to contact birds was also confirmed. The findings show that three passerine species; common reed buntings, brown-eared bulbuls and pale thrushes are susceptible to infection by H5N1 HPAI viruses, which emphasizes that continued surveillance of species other than waterfowl is crucial for effective monitoring of H5N1 HPAI virus outbreaks.
With 5,000 passerine species and dozens of HPAI viruses in circulation around the globe, research into their interactions has only just begun. The evidence to date, while limited, suggests that passerine birds may play a bigger role in the spread of avian flu than previously suspected.